This invention relates to a method for the selective hydrodealkylation and transalkylation of aromatic hydrocarbons. Alkyl aromatic compounds have long been produced from hydrocarbon fractions relatively rich in such materials. Early sources were liquids from cooking or other distillation of coals. More recently, these products have been derived from fractions obtained in refining of petroleum. An important source in recent years has been the aromatic liquid naphthas resulting from the thermal cracking of gases and naphthas to produce olefins.
However, derived, these aromatic-rich streams containing a broad range of components have usually been distilled and otherwise separated (e.g., solvent extraction) to obtain the desired product components. The purpose of these operations typically has been to obtain para-xylene and benzene which are now used in huge quantities in the manufacture of terephthalic acid and other chemical products. The separated streams resulting from the above separation by distillation or other means accordingly consist of product streams of benzene, toluene, C.sub.8 aromatics containing xylenes and a bottoms product of C.sub.9 and C.sub.10 + aromatics. The C.sub.9 component can be separated by means of distillation and can be a source material for manufacture of lighter aromatic hydrocarbons by hydrocracking but with some attendant material losses. The C.sub.10 component is useful for heavy solvents and gasoline.
The presence of ethylbenzene in mixed xylenes is detrimental to process yields and process economics when these xylenes are utilized in the production of p-xylene. Fractional distillation to remove ethylbenzene from mixed xylenes is not economically practical because of the closeness of their boiling points. Ethylbenzene can be removed from xylenes by repeated recrystallizations but this is economically very expensive and is technically difficult.
This invention relates to a conversion process for the selective hydrodealkylation and transalkylation of fractionated heavy reformate containing ethyltoluenes and propylbenzenes into more useful compounds. More specifically, this invention is concerned with a conversion process for the concurrent transalkylation and hydrodealkylation of a fractionated heavy reformate stream containing ethyltoluenes and propylbenzenes to produce ethylbenzene-lean xylenes, benzene, and C.sub.2 -C.sub.4 paraffins without hydrodealkylating the trimethylbenzenes, toluene or xylenes, utilizing a catalyst comprising a tungsten/molybdenum component of WO.sub.3 and MoO.sub.3 and an acidic catalyst of 60 (wt)% mordenite and 40 (wt)% catalytically active alumina. Fractionated heavy reformates are reformates from which C.sub.8 aromatics and lighter components have been largely removed.
In the prior art, methods which have been used to produce aromatic chemicals from fractionated heavy reformates utilize a hydrocracking or hydrodealkylation step to convert the C.sub.9 and C.sub.10 + aromatic components to benzene, toluene and C.sub.8 aromatics. The C.sub.6 + paraffins are converted into readily distillable low boiling hydrocarbons of C.sub.5 and lighter. Processes utilizing this principle are described in U.S. Pat. Nos. 3,957,621 and 3,862,254. However, there is no teaching in the prior art that ethylbenzene-lean xylenes, benzene and C.sub.2 -C.sub.4 paraffins can be produced from fractionated heavy reformate in the presence of a catalyst comprising a tungsten/molybdenum component of WO.sub.3 and MoO.sub.3 and an acidic cracking component of 60 (wt)% of mordenite and 40 (wt)% of catalytically active alumina without a separate hydrocracking step. A high yield of xylenes is accordingly obtained from C.sub.9 aromatics as large losses to benzene and toluene are not incurred via hydrocracking. A high yield of C.sub.2 -C.sub.4 paraffins is obtained from the hydrodealkylation of the alkyl aromatics.
Although the transalkylation of toluene and trimethylbenzenes has been widely studied (U.S. Pat. Nos. 3,260,764; 3,527,825; 3,677,973) because of the demand for greater quantities of high purity aromatic hydrocarbons, the results of such studies have not been sufficient to cause supplies of these hydrocarbons to increase sufficiently to meet this demand. One of the sources of C.sub.9 aromatics can be the heavy reformate stream; however, the trimethylbenzene concentration in heavy-reformate derived C.sub.9 aromatics often is only 50-60%. The remaining C.sub.9 aromatics content can consist of 35-42% ethyltoluenes and 6-10% propylbenzenes and indane. The presence of ethyltoluenes in a transalkylation reaction feed can have a detrimental effect on both xylene yield and quality, because they would contribute to the formation of ethylxylenes and ethylbenzenes. Equilibrium calculations, based on free energy data, indicate that if heavy-reformate derived C.sub.9 aromatics are used in a transalkylation reaction with toluene, the resulting product will contain as much as 6% ethylxylenes and as much as 13% ethylbenzene in the C.sub.8 aromatics. Accordingly, a process using heavy-reformate derived aromatics feedstock requires a dual-function catalyst, one that possesses deethylation as well as transalkylation capability.
Typical of the prior art on hydrodealkylation of alkyl aromatics are the following:
U.S. Pat. No. 2,422,673 teaches hydrodealkylation or demethylation of an alkyl aromatic using a catalyst containing nickel or cobalt on diatomaceous earth. Temperatures used in the process are between 350.degree.-650.degree. F. and pressures are between subatmospheric to 1000 psig. The reaction is carried out at a low pressure of hydrogen so as to obtain a high proportion of demethylation and a relatively small amount of hydrogenation of aromatic hydrocarbons to naphthenic hydrocarbons.
U.S. Pat. No. 2,734,929 discloses hydrodealkylation of alkyl aromatics, including a process for removing methyl groups which are attached directly to the benzene ring, which methyl groups are more difficult to remove than splitting a longer-chained alkyl group down to a methyl group or removing the longer-chained alkyl group entirely. Examples are toluene and xylene with benzene and toluene resulting respectively. Selective dealkylation of ethylbenzene, m-xylene and p-xylene is disclosed, ethylbenzene being the most readily dealkylated, meta-xylene next and para-xylene the least. The patent teaches that alkyl groups in excess of a single methyl group on the benzene ring are much more easily removed than the last methyl group. According to the patent, the catalyst used contains a Group VI-B or Group VIII metal hydrogenation component such as chromium, molybdenum, tungsten, uranium, iron, cobalt, ruthenium, rhodium, palladium, osmium, iridium and platinum, platinum being the least preferred. The hydrogenation catalyst is preferably suspended on a carrier such as alumina, silica gel, zirconia, thoria, magnesia, titania, montmorillonite clay, bauxite, diatomaceous earth, crushed porcelain. The alumina carrier can also contain some silica. Operating conditions include a temperature between 900.degree.-1200.degree. F. at a pressure of 150-2000 psig.
U.S. Pat. No. 3,478,120 discloses a process for selective hydrodealkylation of ethylbenzene to toluene, benzene, methane and ethane with the hydrodealkylation being carried out in the presence of xylenes. The catalyst used comprises an iron group metal on calcium aluminate. Operating conditions include a temperature range of 500.degree.-1200.degree. F. pressure from atmospheric to 2000 psig.
Accordingly, it is well known in the prior art to hydrodealkylate and/or hydroisomerize alkyl aromatics but concurrent selective hydrodealkylation and transalkylation of ethyltoluene and propylbenzene streams into ethylbenzene-lean xylenes, benzene and paraffins without hydrode-methylating the trimethylbenzenes, toluene or xylenes also present in the stream has not been known.